Current technologies for fabricating semiconductor integrated devices have led to a large reduction in the circuit area requirements of individual electronic devices. In the case of a FET, this is achieved by progressively reducing the size of the FET active areas that compose the basic blocks of most electronic circuits. Sub-micron size (e.g., 0.18-micron) transistors can be currently obtained, and the scaling-down trend shows no signs that it may end there with the integration process.
All types of semiconductor circuits have benefited from the intensified integration, and especially so integrated memory circuits having non-volatile memory cells, such as EPROM, EEPROM and Flash-EEPROM cells, integrated therein.
There are two types of non-volatile memories which are comprised of floating-gate transistors. The first type is represented by EPROMs and can be programmed electrically and erased optically.
The second type, represented by EEPROMs and Flash-EEPROMs, allows the stored information to be modified electrically for both writing and erasing.
In either cases, the information is recorded in the memory in the form of electric charges that are stored into the floating gates of the transistors.
EEPROMs will be specifically considered here, in which the state of any memory cell, or of the floating-gate transistor comprising the cell, can be altered by causing electrons to flow through a thin layer of silicon oxide by tunnel effect.